Vacuum flask



FIG. 1 is a perspective view of a first embodiment of the vacuum flask, showing my design thereof;

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a top view thereof;

FIG. 7 is a bottom view thereof;

FIG. 8 is a perspective view of a second embodiment of the vacuum flask, showing my design thereof;

FIG. 9 is a front view thereof;

FIG. 10 is a back view thereof;

FIG. 11 is a right side view thereof;

FIG. 12 is a left side view thereof;

FIG. 13 is a top view thereof; and,

FIG. 14 is a bottom view thereof. 

The ornamental design for a vacuum flask, as shown and described. 